1. Field of the Invention
This invention relates to a toner to be used for elecrophotography, electrostatic printing and, in particular to a toner suitable for the heat fixing method.
2. Description of the Prior Art
In the prior art, there have been known a number of electrophotographic methods as disclosed in U.S. Pat. No. 2,297,691, Japanese patent publication No. 23,910/1967 and Japanese patent publication No. 24,748/1968. In general, photoconductive substances are utilized, and electrical latent images are formed on a photosensitive member by various means. The development of said images is effected by use of a toner, and after optionally carrying out transfer of the toner images to transfer materials such as paper, the images are fixed by heating or pressing to obtain copied products.
Various developing methods have been known, in which electrical latent images are visualized by use of a toner.
For example, there have been known a large number of developing methods such as the magnetic brush method disclosed in U.S. Pat. No. 2,874,063; the cascade developing method disclosed in U.S. Pat. No. 2,618,552; the powder cloud method and the fur brush method disclosed in U.S. Pat. No. 2,221,776; the liquid developing method; and so on.
As the toner for these developing methods, there have been employed fine powders having dispersed dyes or pigments in natural or synthetic resins. Further, it is also known to use fine developing powders in which third substances have been added for various purposes.
The developed toner images are fixed, after being transferred onto transfer materials such as paper, if necessary.
As methods for fixing toner images, there is the method in which the toner is melted under heating by means of a heater or a hot roller; the method in which a binder resin in the toner is softened or dissolved by an organic solvent to be fixed onto a support; and the method in which the toner is fixed onto the support by pressing.
Toners are prepared from materials selected to be suitable for the respective fixing methods, and a toner to be used for a specific fixing method cannot generally be used for other fixing methods. In particular, it is almost impossible to use a toner to be used for the heat fusion fixing method in the hot roller fixing method, the solvent fixing method or the pressure fixing method. Accordingly, toners have been studied and developed to be suited for the respective fixing methods.
There have been also known various magnetic recording methods, in which magnetic latent images are formed and developed with magnetic toners.
Concerning the step of fixing toner images onto papers or other materials, various methods and devices have been developed. The most common method presently available is the so called hot roll fixing system which applies heat and pressure at the same time. In this system an image receiving sheet carrying toner images is brought into contact with heated rollers to have the toner images fixed onto the image receiving sheet. However, when such a fixing system is utilized, the toner of the prior art suffers from troubles such as the so called off-set. Off-set is an unfavorable phenomenon in which a part of the toner carried on the image receiving sheet is transferred onto the roller surface.
As described in Japanese patent publication No. 23,354/1976, such an off-set phenomenon, particularly hot off-set, is likely to occur when a low molecular weight resin is used. Therefore, as also described in said Patent Publication, off-set phenomena may be prevented to some extent by use of a crosslinked resin. However, as a matter of course, by utilizing merely a crosslinked resin, the fixing temperature is elevated, whereby unfixed portions may be formed under ordinary fixing conditions, thus resulting in the problem of low temperature off-set at unfixed portions.
The rollers that contact with the toner images are formed at least at their surface layers of silicone rubber or fluorine containing resin having good releasability. Alternatively, there is also a system in which a release oil such as silicone oil is applied on the roller surfaces to prevent off-set as well as to prevent fatigue of the roller surfaces. However, in the system, in which an oil is applied, there are problems such as the fixing device becoming complicated by the provision of an oil applying system, and also an unpleasant feeling is given to users by the vaporization of the oil. Therefore, prevention of off-set by oil coating is not preferable. Under the present invention, however, a toner having good off-set resistance with a broad fixing temperature region has been developed. Also, as a matter of course, toners are required to be excellent in their anti-blocking property, developing characteristic, transfer characteristic, cleaning characteristic, etc. in addition to their fixing characteristic. Nevertheless, the toners of the prior art possess one or more of the defects mentioned below. That is, most of the toners which can be easily melted by heating are likely to cake or agglomerate in a copying machine. Most of the toners tend to deteriorate in their triboelectric characteristics and free flowing property due to moisture in the environment. Also, in most toners, image densities obtained change, or the background densities increase to lower the quality of the copied products because of mutual deteriorations of toner particles, carrier particles and the photosensitive plate resulting from collision between the toner particles and carrier particles and the contact thereof with the surface of the photosensitive plate during repeated developments and continuous usage. Accordingly, it has been desired to have a toner which is excellent in various toner characteristics and suitable for use in the hot roller fixing method.
Further, in recent years, for more efficient copying operations, high speed fixing has been becoming popular. There is an attempt to lower the softening point of a binder resin in the toner to thereby effect easy heat fixing, for the purpose of increasing the fixing speed in the heat fixing system of the prior art. But; lowering of the softening point will cause troubles such as the occurrence of agglomeration or blocking of toner particles during usage. Thus, it has been strongly desired to have a toner which is suitable for higher speed hot roller fixing, which is also excellent in toner characteristics and which does not cause roller off-set and agglomeration or blocking of the toner particles.
We have made studies and researches on the materials which are good in hot roll high speed fixing characteristic and also excellent in off-set resistance. Consequently we found that polyester resins satisfy such requirements to a considerable extent. Further, among polyester resins, amorphous resins having network structures were found to be preferable in off-set resistance. However, even such polyesters have many problems with respect to high speed fixing characteristic, off-set resistance, image characteristics at high humidity, etc. As the result of extensive efforts, it has been found that a combination of a specific polyester with a crosslinking agent can satisfy significantly the above characteristics. The present invention has been accomplished based on such a finding.